Dual-time-point FDG-PET/CT Imaging of Temporal Bone Chondroblastoma: A Report of Two Cases.

Temporal bone chondroblastoma is an extremely rare benign bone tumor. We encountered two cases showing similar imaging findings on computed tomography (CT), magnetic resonance imaging (MRI), and dual-time-point (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET)/CT. In both cases, CT images revealed temporal bone defects and sclerotic changes around the tumor. Most parts of the tumor showed low signal intensity on T2-weighted MRI images and non-uniform enhancement on gadolinium contrast-enhanced T1-weighted images. No increase in signal intensity was noted in diffusion-weighted images. Dual-time-point PET/CT showed markedly elevated (18)F-FDG uptake, which increased from the early to delayed phase. Nevertheless, immunohistochemical analysis of the resected tumor tissue revealed weak expression of glucose transporter-1 and hexokinase II in both tumors. Temporal bone tumors, showing markedly elevated (18)F-FDG uptake, which increases from the early to delayed phase on PET/CT images, may be diagnosed as malignant bone tumors. Therefore, the differential diagnosis should include chondroblastoma in combination with its characteristic findings on CT and MRI.

However, to the best of our knowledge, there have been no reports on the features of this tumor, based on dual-time-point 18 F-FDG PET/CT images.
Herein, we present two cases of temporal bone chondroblastoma, showing markedly elevated 18 F-FDG uptake which increased from the early to delayed phase on PET/CT images, suggesting the need for caution against the misdiagnosis of these tumors as malignant tumors.

Case 1
A 32-year-old male patient presented with hearing loss in the left ear. MRI was performed at a hospital to explore the cause of patient's symptoms. As the images indicated, a left temporal bone tumor was suspected, and the patient was referred to our hospital for further evaluation and treatment.
The patient underwent CT, MRI, and dual-timepoint 18 F-FDG PET/CT imaging at our hospital ( Figure 1). The CT scan revealed temporal bone defects and secondary otitis media, caused by the tumor; sclerotic changes were observed in the surrounding bones (Figure 1 a-c). On T2-weighted MRI images, most of the tumor showed low signal intensity, while some components showed high signal intensity (Figure 1d).
Gadolinium contrast-enhanced T1-weighted images revealed non-uniform enhancement ( Figure 1f). No increase in signal intensity was noted on diffusion-weighted images ( Figure 1g). The tumor showed markedly elevated FDG uptake on 18 F-FDG PET/CT images (Figures 1h and 1i). The maximum standardized uptake values (SUV max ) in the early (1h) and delayed phases (2h) were 17.1 and 20.2, respectively.
The retention index (RI) was estimated at 18.1%, using the following formula: RI=(delayed SUV max -early SUV max )/early SUV max ×100 (%) The microscopic examination of biopsy specimens revealed that the tumor was mainly composed of uniform, round to polygonal cells and numerous randomly-distributed osteoclasttype giant cells. Considering the tumor site, the preoperative diagnosis was giant cell reparative granuloma, and complete tumor resection was performed. Histopathologic examination of the resected specimen revealed that the tumor contained a chondroid matrix and hemosiderin deposition, which suggested tumor hemorrhage ( Figure 2a).  obtained results, the tumor was diagnosed as a temporal bone chondroblastoma. Although these evaluations are not routinely applied, further analyses revealed weak expressions of both glucose transporter-1 (Glut-1) and hexokinase II (HK-II) (Figures 2c and 2d).

Case 2
A 22-year-old male patient had undergone myringotomy several times within the previous year for recurrent otitis media with effusion in the left ear. MRI was performed at another hospital to explore the cause of recurrent episodes, and left temporal bone tumor was discovered. The patient underwent CT, MRI, and dual-time-point 18 F-FDG PET/CT at our hospital ( Figure 3).
The findings of these imaging modalities were quite similar to those reported in case 1. However, in this case, signal intensity within the tumor in T2-weighted images was slightly lower than that reported in case 1. The SUV max values in the early and delayed phases on 18 F-FDG PET/ CT images were 17.3 and 19.4, respectively, and the RI was 12.1%. Tumor biopsy was performed. The preoperative diagnosis was a giant cell  tumor, based on which the tumor was completely removed. The histopathological findings were similar to those in case 1 and the final diagnosis was temporal bone chondroblastoma.

Discussion
Chondroblastoma is a rare, benign, cartilageproducing neoplasm, which is typically composed of round or polygonal chondroblasts, osteoclasttype giant cells, and a bluish to eosinophilic chondroid intracellular matrix with focal calcification (7). This tumor predominantly arises from the epiphyses of long bones. Only around 80 cases of temporal bone chondroblastoma have been reported (2).
It is known that chondroblastoma often shows increased 18 F-FDG uptake in spite of its benign pathological features (6). Hamada et al. reported the case of an ischial chondroblastoma with increased 18 F-FDG uptake (7). Interestingly, in the mentioned cases and our two patients, immunohistochemical analysis demonstrated only weak expression of Glut-1 and HK-II. Red blood cells were properly marked, though other cell types were not stained.
Few lymphocytes were scattered; thus, inflammation is unlikely to be the cause of FDG accumulation. The precise mechanism of increased 18 F-FDG uptake in chondroblastoma is still unknown. However, considering the low signal intensity in T2-weighted MRI images and hemosiderin deposition in the resected specimen, tumor hemorrhage may contribute to some 18 F-FDG pooling.
On the other hand, there have been no reports evaluating the changes in 18 F-FDG uptake from the early to the delayed phase in chondroblastoma. However, Tian et al. indicated that dual-time-point 18 F-FDG PET may be helpful for differentiating malignant from benign bone tumors (8); it should be mentioned that patients with chondroblastoma were not included in their study. They showed that the sensitivity, specificity, and accuracy of 18 F-FDG PET for such discrimination were 90.6%, 76.0%, and 83.7%, respectively, when 10% RI was considered as the cut-off point.
Our two presented cases would be labeled false-positive if the mentioned criteria were to be adopted. We should bear in mind that the 18 F-FDG uptake, which increases from the early to the delayed phase in chondroblastoma, can result in misdiagnosis of this tumor as a malignancy.
Temporal bone chondroblastoma appears as a well-defined osteolytic mass with mild enhancement on CT images. Calcification within the lesion has been documented in 20-50% of cases (3). On MRI, chondroblastoma is generally visualized as low signal intensity on T1-weighted images and as low-to-high signal intensity on T2-weighted images. Gadolinium-enhanced MRI reveals heterogeneous enhancement of the solid parts of the tumor. Some tumors also contain cystic components (4).
To the best of our knowledge, the features of these tumors on diffusion-weighted images have not been reported. Based on the findings in our case report, chondroblastoma is not likely to show elevated signal intensity on diffusion-weighted images.
Our two cases were preoperatively diagnosed as giant cell reparative granuloma and giant cell tumor, respectively. Osteoclast-like, multinucleated giant cells are histological common findings in chondroblastoma, giant cell reparative granuloma, and giant-cell tumors (5,9,10). Since the biopsy specimens in our cases did not reveal the presence of a chondroid matrix, accurate diagnosis was difficult at that time.
In the surgical specimen, in addition to the detection of a chondroid matrix, immunohistochemical analysis of S-100 expression assisted us in making the final diagnosis of chondroblastoma. Demonstration of S-100 positivity in chondroblasts is helpful in differentiating chondroblastoma from giant cell reparative granuloma and giant cell tumors (5,10).
To the best of our knowledge, there have been no reports indicating 18 F-FDG PET/CT findings of giant cell reparative granuloma. On the other hand, giant cell tumors are known to show markedly elevated 18 F-FDG uptake (6,8). In fact, it is difficult to discriminate between chondroblastoma and giant cell tumors, based on the level of 18 F-FDG uptake.
In conclusion, we reported two cases of temporal bone chondroblastoma. Based on the findings, when temporal bone tumors with markedly elevated 18 F-FDG uptake, which increases from the early to the delayed phase on PET/ CT images, are encountered, chondroblastoma with its specific CT and MRI features should be considered in the differential diagnosis.

Conflicts of interest
No potential conflicts of interest were disclosed.